The present invention relates to a gas turbine combustor and, more particularly, to a gas turbine combustor which is suitable for reduction in emission of nitrogen oxides (hereunder referred to as NOx).
NOx which occurs at time of combustion of natural gas, kerosene, gas oil (light oil), etc. is thermal NOx and occurs through oxidization of nitrogen in the air. The occurrence of the thermal NOx depends highly on temperature. In general, in a gas turbine in which low nitrogen content fuel is used, reduction of flame temperature is a principal concept of low NOx combustion. A combustor of a gas turbine is different from a burner used in boilers, etc.. That is, the fuel flow rate changes according to the gas turbine load. On the other hand, air flow rate is substantially fixed, and the fuel air ration, which is a mass flow ration between fuel and air, changes greatly between a partial load and a 100% rated load. Further, at the rated load, in which the fuel flow rate is a maximum, a lot of air, up to twice as much as the theoretical air flow rate necessary to effect complete combustion of fuel, is supplied. Therefore, a lean burn which forms a flame with much excess air can be employed, and the lean burn is a leading technology in a method of reduction of NOx adapted in the current combustor.
Combustion methods of gas fuel are classified into a diffusion combustion method of burning fuel while mixing the fuel with air and a premixed combustion method of premixing fuel and air and then jetting the mixture from a nozzle to burn it. The diffusion combustion method is excellent in stability of the flame and is able to form a flame in a wide range of fuel air ratios. However, since the fuel is burned while being mixed with air, the fuel air ration changes greatly, spatially, within the flame, so that even if lean burn is tried, a part of the fuel is burned under the condition of fuel rich combustion. Therefore, the flame temperature is raised partially and a lot of NOx is apt to occur.
In premixed combustion, fuel and air are mixed before they are introduced into the combustion chamber. Therefore, with the premixed combustion method it is easier to provide a uniform fuel air ration within the flame than it is with diffusion combustion, and the formation of partial temperature elevation due to lock of uniformity of mixing can be avoided, so that an effect of reduction of NOx is large. However, the fuel air ratio range in which the flame is stably formed and the conditions of jetting velocity are narrower than with diffusion combustion. In particular, in an operation of a turbine from starting to 100% load or operation of load interruption at emergency, since the fuel flow rate changes widely, it is difficult to operate the gas turbine only by premixed combustion, so that a two step combustion method is employed in which diffusion combustion is effected from the staring to a partial load and then premixed combustion is started when the turbine goes beyond the partial load. For combustors employing the two step combustion method, there are two kinds, one of which is a combustor in which combustion chambers are provided independently for diffusion combustion and for premixed combustion, respectively, which is disclosed in JP A 61-22106 and JP A 61-22127, for example. An example of the other kind is a combustion in which the diffusion combustion and the premixed combustion are effected in the same combustion chamber, which is disclosed in JP A 63-161318.
Further, since the fuel air ratio increases and the flame temperature elevates according to an increase in the gas turbine load or an increase in the fuel flow rate, NOx increases according to the increase in load. When the two step combustion method is employed, the NOx emission amount can be reduced after staring of the premixed combustion, but the NOx occurrence amount increases during the diffusion combustion before starting of the premixed combustion. In order to suppress NOx occurrence over a wide load range, the premixed combustion is started at a load as low as possible, but it is necessary to suppress NOx occurrence in the diffusion combustion.
In order to form a premixed flame, it is necessary to set the injection velocity of the mixture and the fuel air ratio within a range. When the injection velocity becomes larger and the fuel air ratio becomes smaller, the flame is blown out, and when the injection velocity becomes lower and the fuel air ratio becomes larger, the flame comes into the mixture injection nozzle, and so-called back fire takes place. The amounts of fuel and air necessary to form the flame are determined by the outlet diameter of the nozzle. If the outlet diameter of the nozzle is made larger, the amount of fuel and air necessary to inject at a velocity necessary for stable combustion increases, and a gas turbine load at which a premixed combustion can be effected becomes higher. If the outlet diameter of the nozzle is made smaller, a premixed flame can be formed stably by a small amounted of fuel and air, but an amount of fuel in which the premixed combustion can be effected becomes small and a ratio of the amount of fuel to be burned by the diffusion combustion increases, so that the amount of NOx occurrence increases. Therefore, when it is intended to reduce NOx during a high load operation, the premixed nozzle is made larger, so that a gas turbine load at which the gas turbine is operated by premixed combustion becomes high.
As an example of a combustor for addressing the above-mentioned subjects, a combustor provided with a mechanism for adjusting an air flow rate for diffusion combustion and premixed combustion, and reducing NOx, CO, etc. over a wide load range by optimizing the air flow rate distribution is already proposed in JP A 60-91141, JP A 60-218535, JP A 61-153316 and JP A 61-52523, for example. A combustor in which a plurality of premixed nozzles are provided and the number of the premixed nozzles in use is changed according to load also is proposed in JP U 2-100060 (Laid-Open Utility-Model Application), for example.
The gas turbine can be operated with less NOx emission. However, in order to improve further the performance, there is the following subject to be addressed. When the combustion chamber is divided, each of diffusion combustion and premixed combustion is independently effected so that flames can be prevented from interfering with each other and stable flames can be formed. However, combustion air also is divided into air for diffusion combustion flame and air for premixed combustion flame, and a ratio of the premixed combustion can not be increased. In case diffusion flame and premixed flame are formed in the same combustion chamber and the combustion air is used commonly for both diffusion combustion and for premixed combustion, for example, when the gas turbine load is low, there is a problem to be solved as the fuel air ratio in diffusion flame becomes small by air for premixed flame and unburned components become easy to exhausted.
Further, in order to reduce NOx further in a wide range of loads, it is necessary that the premixed nozzle be made large to increase the ratio of the premixed combustion. The premixed combustion is started from a low load, and NOx occurred during the diffusion combustion is made as small as possible.
Of the above-mentioned prior art, in the combustor provided with an air flow adjustment mechanism, when the air amount for premixed combustion is increased in order to effect premixed combustion at a low load, injection velocity of premixed fuel air also becomes low and there is a possibility of back fire occurring. Therefore, in particular, when the premixed nozzle outlet diameter is made large, it is insufficient to reduce the load at which the premixed combustion starts.
On the other hand, in the combustor which is provided with a plurality of premixed nozzles and the number of the nozzles in use is changed according to load, since the construction is such that the plurality of annular premixed nozzles are provided concentrically with an axis, when the number of the premix nozzles in use increases, the next nozzle has to be ignited with a relatively low temperature premixed flame, so that there is a problem that ignitability of the premix nozzle of the second stage or later stages worsen. Further, there are problems such that an area in which flames of adjacent premixed nozzles contact each other is large, the premixed flames interfere with each other, pressure change (combustion vibration) becomes large and the life of the combustor shortens.